I tell him its probably a bag of cocaine, and that I've seen a white powdery substance on the floor once or twice. I say I have no idea who it is, but that he should look for someone who looks messed up after lunch every day.
Put a zip lock bag full of baking soda in his mail box. Tape this note that you printed out - and don't get fingerprints on it.
I know you know I've been partying in the bathroom, here's a little something for keeping your mouth shut. Have a good time.
PS. I always knew you were a partying motherf*cker.